CDMA mobile communication method, system and mobile station apparatus

ABSTRACT

A CDMA mobile communication system, which produces a time for monitoring peripheral base stations using different radio frequencies for carrying out hand-off between base stations using different radio frequencies without momentary cutoff of communication. A mobile station (504) of the CDMA mobile communication system discontinues error correction decoding on reception signal, outputs a user data which is not error correction decoded and, during the non-reception time in the blank time, changes over a reception frequency to a radio frequency of a neighboring base station monitor a reception level of the neighboring base station. During a hand-off period between a base station under communication (502) and a new base station (503) which differ in used frequency, the base station under communication and the new base station transmit only a user data portion, but do not transmit transmission signal in a transmission period for transmitting an error correction coding check bits. The mobile station (504) receives and demodulates the transmission signal of the base station under communication and, during an error correction coding check bit transmission period, changes over a frequency of a receiver to the frequency of the new base station to receive and demodulate the transmission signal of the new base station.

TECHNICAL FIELD

The present invention relates to reception level monitoring and hand-offoperation in cellular mobile communications.

BACKGROUND ART

In cellular mobile communications, a number of base stations aredisposed in a large service area. The base station to be connected issuccessively changed over as the mobile station moves so that the mobilestation can make communication with a base station of the bestcommunication quality to continue the communication. In this case, tosearch for a base station to be next connected, during communicationradio waves of base stations around the base station under communicationare received to measure the reception levels. From the measurementresults, the mobile station determines a neighboring base station thehighest reception level as the new base station to be next connected,and notifies the result to the base station in communication. Using thisprocedure, when change-over (hand-off) of the base station becomesrequired, an upper level station of the base station under communicationinstructs the base station under communication and the new base stationto begin hand-off operation. During the hand-off operation, the sametransmission data is transmitted from the base station undercommunication and the new base station, the mobile station changes overthe transmission/reception frequency from the base station undercommunication to the new base station, or vice versa, in alternation.This is the method of monitoring neighboring base stations and hand-offin cellular mobile communications.

In a time division multiple access (TDMA) communication system, thetransmission time is divided into short times called slots, and a numberof mobile stations are assigned to different slots to make transmissionand reception periodically. Therefore, the time other than thetransmission and reception slot of the mobile station is a blank time.Making use of the blank time, the mobile station allows monitoringneighboring base stations to select a base station easily which providesthe highest reception level. Thus, hand-off is made to the selected newbase station (see "digital mobile telephone system" standardspecifications, RCR STD-27B, by Research & Development Center for RadioSystem, or "Personal digital cellular telecommunication system RCRstandard, RCR STD-27", Research & Development Center for Radio System).

On the other hand, in a code division multiple access (CDMA)communication system, though it has no blank time as seen in TDMA, sinceall of the base stations may use the same radio frequency, it is notnecessary to make neighboring base station monitoring or changing overthe transmission/reception frequency at hand-off operation (see R.Padovani, "Reverse link performance of IS-95 based cellular systems,"IEEE Personal Communications, vol. 1, pp.28-34, 3rd Quarter 1994). Thatis, in CDMA, each base station spreads the transmission data to widebandsignals using different spreading code sequence with a same frequency.Therefore, at the receiving side, it is sufficient to provide anothercorrelator for despreading the received signal with the spreading codesequence of the peripheral base stations in order to achieve neighboringbase station monitoring and hand-off operation.

However, also in CDMA, there is a case where base stations cannot usethe same radio frequency. This is, for example, the case of a macrocellbase station with a wide communication range which is overlaid bymicrocell base station with a narrow communication range. Since thetransmission power of the microcell base station is smaller than that ofthe macrocell base station, if the same radio frequency is used, thetransmission signal of the macrocell base station considerably affectsthe reception signal of the mobile station which is under communicationwith the microcell base station. Further, since the transmission powerof the macrocell mobile station may be higher than the transmissionpower of a mobile station under communication with a microcell basestation, the reception signal of the microcell base station is greatlyinterfered. To avoid such interference, it is necessary to use differentradio frequencies between the microcell and macrocell base stations. Inthis case, the frequency is required to be changed over for neighboringbase station monitoring and hand-off operation when change-over is madefrom the microcell base station to the macrocell base station or viceversa.

Since CDMA has no blank time as seen in TDMA, there is a method in whichthe transmission signal of the mobile station is received by neighboringbase stations to detect the reception level, instead of makingneighboring base station monitoring at the mobile station, to select abase station of the highest level as the new base station. In thismethod, each base station requires a receiver for level measurement, andan increasing number of level measurement receivers become required asthe number of mobile stations under communication increases.Furthermore, large amounts of control processing will be required suchas for level measurement and comparison. Yet further, at hand-offoperation, the communication is momentarily cut off because the radiofrequency is changed over.

DISCLOSURE OF THE INVENTION

With a view to solve the above problems, it is a primary object of thepresent invention to provide a transmission/reception method which, inCDMA mobile communications, has a blank time for measuring the receptionsignals from base stations using different radio frequencies, wherebysignal levels from the neighboring base stations can be monitored inthat time.

Another object of the present invention is to provide atransmission/reception method during hand-off operation which, in CDMAmobile communications, allows making a blank time during communicationin order to carry out hand-off operation between base stations usingdifferent radio frequencies without instantaneous interception.

First, in accordance with the present invention, there is provided aCDMA mobile communication method, in which a first base station, afterthe user data is error correction encoded, transmits a narrow-bandmodulated signal obtained by primary modulation of the error correctioncoded user data, followed by secondary modulation by a spreading code toa wideband signal, and the mobile station makes conversion of thewideband signal to the narrow-band modulated signal, followed bydemodulation and error correction decoding, to reproduce the user data;wherein the mobile station comprises the steps of:

demodulating the user data during a first time of receiving the userdata transmitted from the first base station;

discontinuing the error correction decoding during a second time ofreceiving the error correction check bits and outputting the user datawhich are not error correction decoded; and

changing over, in the second time, the reception frequency to a radiofrequency of the second base station differing in used frequency fromthe first base station to monitor the reception level of the second basestation.

In the CDMA mobile communication method, during the hand-off operationbetween the first base station and the second base station,

the first base station may have a step for transmitting the user data inthe first time and discontinuing the transmission in the second time,

the second base station may have a step for transmitting the same userdata in the second time and discontinuing the transmission in the firsttime,

the mobile station may have a step for receiving and demodulating thesignal from the first base station in the first time, changing over thefrequency of the receiver to the radio frequency of the second basestation in the second time to receive and demodulate the signal from thesecond base station, and recovering the user data by combining thesignal from the first base station and the signal from the second basestation.

In the CDMA mobile communication method, during the hand-off operation,

the mobile station may be provided with a step for transmitting the userdata to the first base station at a first radio frequency in a thirdtime in which the user data is to be transmitted, and transmitting thesame user data to the second base station at a second radio frequency ina fourth time in which the error correction check bits are to betransmitted,

the first base station may be provided with a step for demodulating theuser data received in the third time,

the second base station may be provided with a step for demodulating theuser data received in the fourth time, and

a upper level station of the first base station and the second basestation may be provided with a step for reproducing the user data bycombining their demodulated signals.

In the CDMA mobile communication method, during the hand-off operationperiod between the first base station and the second base station,

the first base station may have a step for transmitting the user data inthe first time and discontinuing the transmission in the second time;

the second base station may have a step for transmitting the errorcorrection check bits in the second time and discontinuing thetransmission in the first time;

the mobile station may have a step for receiving and demodulating thesignal from the first base station in the first time, changing over thefrequency of the receiver to the radio frequency of the second basestation in the second time to receive and demodulate the signal from thesecond base station, and reproducing the user data by using the userdata transmitted from the first base station and error correction checkbits transmitted from the second base station.

In the CDMA mobile communication method, during the hand-off operation,

The mobile station may have a step for transmitting the user data at thefirst radio frequency to the first base station in the third time inwhich the user data is to be transmitted, and transmitting the errorcorrection check bits at the second radio frequency to the second basestation in the fourth time in which the error correction check bits isto be transmitted;

the first base station may have a step for demodulating the user datareceived in the third time;

the second base station may have a step for demodulating the errorcorrection check bits received in the fourth time; and

the upper level station of the first base station and the second basestation may have a step for reproducing the user data by errorcorrection decoding using these demodulated signals.

Secondly, according to the present invention, there is provided a CDMAmobile communication method, in which a first base station, after theuser data is error correction encoded, transmits a narrow-band modulatedsignal obtained by primary modulation with multivalue modulation of theuser data and the error correction check bits, followed by secondarymodulation by a spreading code to a wideband signal, and the mobilestation makes conversion of the wideband signal to the narrow-bandmodulated signal, followed by demodulation and error correctiondecoding, to reproduce the user data; wherein during a reception levelmonitoring period for the mobile station to monitor the reception levelof the signal from a second base station differing in frequency from thefirst base station:

the first base station comprises a step for making a second time fordiscontinuing transmission by transmitting the user data and the errorcorrection check bits in the first time with an increased level of themultilevel modulation;

the mobile station comprises a step for demodulating the received signalwith an increased level of the multilevel modulation and making errorcorrection decoding to reproduce the user data in the first time,changing over the reception frequency to the radio frequency of thesecond base station to monitor the reception level of the second basestation in the second time.

In the CDMA mobile communication method, during the hand-off operationperiod between the first base station and the second base station,

the first base station may have a step for transmitting the user dataand its error correction check bits in the first time;

the second base station may have a step for transmitting the user dataand its error correction check bits in the second time;

the mobile station may have a step for receiving the user data and theerror correction check bits from the first base station in the firsttime, changing over the reception frequency to the radio frequency ofthe second base station to receive the user data and the errorcorrection check bits in the second time, and reproducing the user databy combining the signal from the first base station and the signal fromthe second base station.

Thirdly, according to the present invention, there is provided a CDMAmobile communication system, comprising a first base station which,after the user data is error correction encoded, transmits a narrow-bandmodulated signal obtained by primary modulation of the user data and theerror correction check bits, followed by secondary modulation by aspreading code to a wideband signal, and a mobile station for makingconversion of the wideband signal to the narrow-band modulated signal,followed by demodulation and error correction decoding, to reproduce theuser data; wherein the mobile station comprises:

a demodulator for demodulating the user data in a first time ofreceiving the user data transmitted from the first base station;

an error correction decoder which discontinues the error correctiondecoding in the second time of receiving the error correction check bitsand outputs the user data which are not error correction decoded;

frequency change-over means for changing over the reception frequency toa radio frequency of the second base station differing in used frequencyfrom the first base station in the second time; and

a received level measuring unit for monitoring the reception level ofthe signal from the second base station.

The CDMA mobile communication system may comprise an upper level stationfor instructing hand-off between the first base station and the secondbase station, wherein

the first base station may comprise first control means for transmittingthe user data in the first time and discontinuing the transmission inthe second time according to the hand-off instruction from the upperlevel station;

the second base station may comprise second control means fortransmitting the user data in the second time and discontinuing thetransmission in the first time according to the hand-off instruction;and

the mobile station may comprise receiving frequency switching means fortuning the frequency of a receiver to one of the transmission frequencyof the first base station and the transmission frequency of the secondbase station, and a demodulator for receiving and demodulating thesignal from the first base station in the first time and receiving anddemodulating the signal from the second base station in the second timefor combining the signal from the first base station and the signal fromthe second base station to reproduce the user data.

In the CDMA mobile communication system,

the mobile station may comprise transmission frequency change-over meansfor changing over the transmission frequency of a transmitter betweenthe reception frequency of the first base station and the receptionfrequency of the second base station, and transmission means for, duringthe hand-off period, transmitting user data at the reception frequencyof the first base station in the third time in which the user data is tobe transmitted and transmitting the user data at the reception frequencyof the second base station in the fourth time in which the errorcorrection check bits are to be transmitted;

the first base station may comprise a first demodulator for demodulatingthe user data received in the third time;

the second base station may comprise a second demodulator fordemodulating the user data received in the fourth time; and

the upper level station may comprise means for reproducing the user databy combining the demodulated signals demodulated by the first and secondbase station demodulators.

The CDMA mobile communication system may comprise a upper level stationfor instructing hand-off between the first base station and the secondbase station, and the first base station may comprise first controlmeans for transmitting the user data in the first time and discontinuingthe transmission in the second time according to the hand-offinstruction from the upper level station;

the second base station may comprise second control means fortransmitting the error correction check bits in the second time anddiscontinuing the transmission in the first time according to thehand-off instruction;

the mobile station may comprise receiving frequency switching means forchanging the frequency of the receiver between the transmissionfrequency of the first base station and the transmission frequency ofthe second base station, and a demodulator for receiving anddemodulating the signal from the first base station in the first timeand receiving and demodulating the signal from the second base stationin the second time to reproduce the user data using the user data fromthe first base station and the error correction check bits from thesecond base station.

In the CDMA mobile communication system,

the mobile station may comprise transmission frequency change-over meansfor changing over the transmission frequency of a transmitter betweenthe reception frequency of the first base station and the receptionfrequency of the second base station, and transmission means for, duringthe hand-off period, transmitting user data at the reception frequencyof the first base station in the third time in which the user data is tobe transmitted and transmitting the error correction check bits at thereception frequency of the second base station in the fourth time inwhich the error correction check bits are to be transmitted;

the first base station may comprise a demodulator for demodulating theuser data received in the third time;

the second base station may comprise a demodulator for demodulating theerror correction check bits received in the fourth time; and

the upper level station may comprise means for making error correctiondecoding using the demodulated signals outputted from the individualdemodulators to reproduce the user data.

Fourthly, according to the present invention, there is provided a CDMAmobile communication system, comprising a first base station which,after the user data is error correction encoded, transmits a narrow-bandmodulated signal obtained by primary modulation with multilevelmodulation of the user data and the error correction check bits,followed by secondary modulation by the spreading code to a widebandsignal, and a mobile station for making conversion of the widebandsignal to the narrow-band modulated signal, followed by demodulation anderror correction decoding, to reproduce the user data; wherein

the first base station comprises a first modulation level control unitfor varying the level of the multilevel modulation of the user data andthe error correction check bits, and means for making a second time fordiscontinuing transmission by transmitting with an increased modulationlevel in the first time during a reception level monitoring period forthe mobile station to monitor the reception level of the signal from asecond base station differing in frequency from the first base station;and

the mobile station comprises receiving frequency switching means forchanging the frequency of the receiver between the transmissionfrequency of the first base station and the transmission frequency ofthe second base station, a second modulation level control unit forchanging the level of the multilevel modulation, and a demodulator fordemodulating the reception signal with an increased modulation level andmaking error correction decoding to reproduce the user data, and areceived level measuring unit for changing over the reception frequencyto the transmission frequency of the second base station to monitor thereception level of the second base station.

The CDMA mobile communication system may comprise a upper level stationfor instructing hand-off between the first base station and the secondbase station, and the first base station may comprise first transmissionmeans for transmitting the user data and its error correction check bitsin the first time according to the hand-off instruction from the upperlevel station; and wherein

the second base station may comprise second transmission means fortransmitting the user data and its error correction check bits in thesecond time according to the hand-off instruction; and

the mobile station may comprise means for receiving the user data andthe error correction check bits from the first base station in the firsttime, changing over the reception frequency to the transmissionfrequency of the second base station in the second time to receive theuser data and the error correction check bits, combining the signal fromthe first base station and the signal from the second base station, anderror correction decoding to reproduce the user data.

Fifthly, according to the present invention, there is provided a CDMAmobile communication system, comprising a first base station which,after the user data is error correction encoded, transmits a narrow-bandmodulated signal obtained by primary modulation of the user data and theerror correction check bits, followed by secondary modulation by aspreading code to a wideband signal, and a mobile station for makingconversion of the wideband signal to the narrow-band modulated signal,followed by demodulation and error correction decoding, to reproduce theuser data; wherein a receiver of the mobile station comprises:

a demodulator for demodulating the user data in a first time ofreceiving the user data transmitted from the first base station;

an error correction decoder which discontinues the error correctiondecoding in the second time of receiving the error correction check bitsand outputs the user data which are not error correction decoded;

a frequency switching unit for changing over the reception frequency toa radio frequency of the second base station differing in used frequencyfrom the first base station in the second time; and

a received level measuring unit for monitoring the reception level ofthe second base station.

Alternatively, in the mobile station, the receiver of the mobile stationmay comprise:

a reception frequency switching unit for changing the receptionfrequency of the receiver between the transmission frequency of thefirst base station and the transmission frequency of the second basestation, and

a demodulator which, during the hand-off period between the first basestation and the second base station, receives and demodulates the signalfrom the first base station in the first time, receives and demodulatesthe signal from the second base station in the second time for combiningthe signal from the first base station and the signal from the secondbase station to reproduce the user data;

and the transmitter of the mobile station may comprise:

transmission frequency change-over means for changing the transmissionfrequency between the reception frequency of the first base station andthe reception frequency of the second base station; and

transmission means for, during the hand-off period, transmitting userdata at the reception frequency of the first base station in the thirdtime in which the user data is to be transmitted and transmitting theuser data at the reception frequency of the second base station in thefourth time in which the error correction check bits are to betransmitted; and wherein

the first base station may have a step for demodulating the user datareceived in the third time;

the second base station may have a step for demodulating the user datareceived in the fourth time; and

a upper level station of the first base station and the second basestation may have a step for combining these demodulated signals toreproduce the user data.

Sixthly, according to the present invention, there is provided a mobilestation in CDMA mobile system, comprising a first base station which,after a user data is error correction encoded, transmits a narrow-bandmodulated signal obtained by primary modulation with multilevelmodulation of the user data and the error correction check bits,followed by secondary modulation by the spreading code to a widebandsignal, and a mobile station for making conversion of the widebandsignal to the narrow-band modulated signal, followed by demodulation anderror correction decoding, to reproduce the user data, the first basestation transmits the user data and the error correction check bits in afirst time with an increased level of the multilevel modulation to makea second time for discontinuing transmission, wherein a receiver of themobile station comprises:

a modulation level controller for increasing the level of the multilevelmodulation in the first time during reception level monitoring periodfor monitoring the reception level of the signal from a second basestation differing from the first base station;

means for reproducing said user data which, during the first time,demodulates said user data and error correction check bits, and errorcorrection decodes said user data.

a reception frequency switching unit for tuning the reception frequencyof the receiver to one of the transmission frequency of the first basestation and the transmission frequency of the second base station; and

a received level measuring unit for monitoring the reception level ofthe second base station in the second time.

In the mobile station, the reception frequency switching unit of thereceive of the mobile station may, during the hand-off period betweenthe first base station and the second base station, changes over thereception frequency to the transmission frequency of the first basestation in the first time, and to the transmission frequency of thesecond base station in the second time, and the reproduction means maycombine the signal from the first base station and the signal from thesecond base station to reproduce the user data.

With the present invention, in CDMA mobile communications, since themobile station is able to make a blank time for measuring the signallevel from a base station using a different radio frequency, the mobilestation can monitor the base station in that time.

Still further, in hand-off operation between base stations usingdifferent radio frequencies in CDMA communications, since, during thehand-off period of the base station and the mobile station, signals ofboth base stations can be received and the same transmission data can betransmitted to both base stations, occurrence of instantaneousinterception in communication can be prevented during hand-off period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are block diagrams showing a first embodiment of atransmitter of a base station and a receiver of a mobile station of theCDMA mobile communication system in accordance with the presentinvention;

FIG. 2 is a timing chart of reception level measurement in the mobilestation in the first embodiment;

FIGS. 3A and 3B are block diagrams showing a second embodiment of atransmitter of a base station and a receiver of a mobile station of theCDMA mobile communication system in accordance with the presentinvention;

FIG. 4 is a timing chart of reception level measurement in the mobilestation in the second embodiment;

FIG. 5 is a block diagram showing the system construction in makinghand-off in third to fifth embodiments of the CDMA mobile communicationsystem in accordance with the present invention;

FIG. 6 is a block diagram showing a transmitter of the mobile station ofthe third embodiment of the CDMA mobile communication system inaccordance with the present invention;

FIG. 7 is a timing chart showing operation of the base station duringhand-off operation in the third embodiment;

FIG. 8 is a timing chart showing operation of the mobile station duringhand-off operation in the third embodiment;

FIG. 9 is a block diagram showing a fourth embodiment of a transmitterof mobile station of the CDMA mobile communication system in accordancewith the present invention;

FIG. 10 is a timing chart showing operation of the base station duringhand-off operation in the fourth embodiment;

FIG. 11 is a timing chart showing operation of the base station duringhand-off operation in a fifth embodiment of the CDMA mobilecommunication system in accordance with the present invention;

FIG. 12 is a timing chart showing operation of mobile station duringhand-off operation in a sixth embodiment of the CDMA mobilecommunication system in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be described withreference to the accompanying drawings.

EMBODIMENT 1

FIGS. 1A and 1B are block diagrams showing a first embodiment of atransmitter of a base station and a receiver of a mobile station of theCDMA mobile communication system in accordance with the presentinvention, FIG. 2 shows a frame structure and reception timing used inthe present invention;

Referring to FIG. 1A, the reference numeral 101 indicates a frameassembler for framing the input data, 102 is a error correction encoderfor making error correction coding the framed data, and 103 is a primarymodulator for making phase modulation of the output of the errorcorrection encoder 102. The numeral 105 indicates a secondary modulatorfor spreading the primary modulated signal with a spreading codeproduced by a spreading code sequence 104. 106 is a frequency converterfor converting the spread signal to a transmission frequency. 107 is atransmission power amplifier for making power amplification. 109 is amain controller for controlling the above-described individual units.

In FIG. 1B, 151 is a frequency converter for converting the receptionsignal to a baseband, 152 is a correlator for despreading the basebandsignal with a spreading code generated by a spreading code sequencegenerator 153. The despread signal is demodulated by a demodulator 154,and reverted back to the original signal by a decision block 157. 158 isan error correction decoder, by which the reverted signal is errorcorrected. 159 is a rate converter, by which the signal is converted tothe complete original signal of the original code rate. 155 is areceived level measuring unit, 156 is a main controller, and 160 is asub-controller. The main controller 156 controls various units of theabove-described receiver. The main controller 156 is inputted withcontrol information extracted from frame data supplied from the errorcorrection decoder 158. The sub-controller 160 is controlled by the maincontroller 156.

The sub-controller 160 comprises a frequency switching unit 162, aspreading code switching unit 164, an enable signal generator 166, and alevel measuring controller 168, whereby controlling change-over ofreception frequency, change-over of spreading code, execution/pausing oferror correction decoding, and timing of reception level measurement.

Operation of the transmission unit of the base station shown in FIG. 1Awill be described. The user data sequence in the base station is firstarranged by the frame assembler 101 to data of each predetermined oneframe time (Tf). Transmission data of 1 frame is error correctionencoded by the error correction encoder 102, disposing the transmissiondata sequence and error correction check bit sequence as shown in FIG.2. To this signal, for example, quadrature phase modulation is carriedout by the primary modulator 103. After that, the quadrature phasemodulated signal is band spread (secondary modulated) by the secondarymodulator 105 to a wideband signal using a spreading code sequence froma spreading code sequence generating unit 104. The wideband signal isfrequency converted by the frequency converter 106 to a radio frequency,and power amplified by the transmission power amplifier 107 to betransmitted.

In the receiver of the mobile station shown in FIG. 1B, first thefrequency converter 151 converts the reception signal to basebandsignal. Then, the correlator 152 correlates (despread) the spreadingcode sequence with the reception signal to obtain a quadrature phasemodulated signal. The spreading code sequence is the same code as usedin transmission from the base station, which is generated by a spreadingcode sequence generator 153. Then, the signal is demodulated by thedemodulator 154, and decision on the transmitted encoded data is made bythe decision block 157. In normal communication (normal mode), detecteddata is error corrected by the error correction decoder 158, and rateconverted by the rate converter 159 to reproduce the data transmittedfrom the base station.

In a neighboring base station monitoring mode, on the other hand, themobile station, as shown in FIG. 2(A), receives only the user datasequence portion of the frame by the function of the subcontroller 160.This control is made as shown below. First, the main controller 156detects data length of the user data from the control data of each framehead portion of the reception signal, and supplies the information tothe sub-controller 160. According to the information, the enable signalgenerator 166 of the sub-controller 160 discontinues error correction bythe error correction decoder 158 during the error correction check bitperiod. Further, during this period, the level measuring controller 168of the sub-controller 160 sends a signal to the received level measuringunit 155 to instruct reception level measurement. Still further, thefrequency switching unit 162 sends a signal to the frequency converter151 to cause the frequency of the receiver to be changed over to thefrequency of the neighboring base station.

FIG. 2(B) shows the receiving frequency switching timing of this time,which uses an error correction code with an coding rate of about 1/2 toenhance the error correction capability (therefore, the user data lengthis about the same as the check bit length). In FIG. 2(A), fo is a radiofrequency of a base station under communication, fk is a radio frequencyof a counterpart base station to be reception level measured. It isshown that during the error correction check bit period, the receptionfrequency is changed over to measure the reception level of aneighboring base station frequency fk. This allows neighboring basestation monitoring without discontinuing the communication.

As described above, error correction decoding is not made during thelevel monitoring period in the present invention. Therefore,deterioration of reliability can be relaxed to some extent by increasingthe transmission power of the transmission data sequence portion at thebase station.

EMBODIMENT 2

FIG. 3 shows another embodiment according to the present invention.FIGS. 3A and 3B are block diagrams showing a second embodiment of aconstruction of the CDMA mobile communication system in accordance withthe present invention. FIG. 3A shows construction of a transmitter of abase station, and FIG. 3B shows construction of a receiver of a mobilestation. FIG. 4 shows frame configuration and reception timing used inthe present invention.

A difference of the transmitter of the base station of FIG. 3A from thetransmitter of the base station shown in FIG. 1A is that the former hasa modulation level controller 304, whereby a primary control unit 303 iscontrolled, for example, from quadrature phase modulation to hexadecimalphase modulation, to enable changeable modulation level.

Further, a difference of the receiver of the mobile station of FIG. 3Bfrom the receiver of the base station shown in FIG. 1B is that theformer has a demodulation level controller 355, whereby a demodulator357 is controlled.

With this construction, in a normal mode, the base station makes 4-levelquadrature amplitude modulation of the transmission signal to betransmitted. However, in a neighboring base station monitoring mode, itmakes 16-level quadrature amplitude modulation, for example. One of fourmodulation states is represented by two bits in the former, whereas oneof 16 modulation states can be represented by 4 bits in the latter.Therefore, 16 level modulated signal transmitted during reception signallevel measurement in the mobile station exists for a half time of frameas shown in FIG. 4(A), and the remnant is a blank time where nomodulated signal is present. That is, a blank time occurs intransmission.

By this method, the frequency of the receiver can be changed over in theblank time without discontinuing communication at the mobile station, asshown in FIG. 4(B), to measure the reception level of a neighboring basestation of frequency fk.

Further, in the monitoring mode period, the transmission power of thebase station can be increased to relax deterioration in reliabilityassociated with increasing the modulation level.

EMBODIMENT 3

In the case of making level monitoring as described in FIGS. 1A and 1Band FIG. 2, hand-off for changing over the base station duringcommunication by using a similar apparatus will be described withreference to FIG. 5 to FIG. 8.

A system configuration for hand-off is shown in FIG. 5. A mobile station504 is under communication with a communicating base station 502. As aresult of measurement, a new base station 503 is better in measurementresult, and it becomes necessary to hand off to the new base station503. In this case, hand-off is made under control of a upper levelstation 501 of the communicating base station 502 and the new basestation 503.

FIG. 6 is a block diagram showing construction of a transmission unit ofthe mobile station 504. In this figure, symbol 60X (X=1-7.9) correspondsto symbol 10X in FIG. 1A.

A difference of this transmission unit from the transmission unit of thebase station shown in FIG. 1A is that the former has a sub-controller620. The sub-controller 620 comprises a transmission data control unit622, a spreading code switching unit 624, and a frequency switching unit626, which individually supplied with control information from a maincontroller 609. The transmission control unit 622 sends a signal to theerror correction encoder 602 to control so that the user data isrepeatedly outputted two times during the period of 1 frame, as shown inFIG. 8(A). Of these two times repeated data, the first data istransmitted to the communicating base station 502, and the next data istransmitted to the new base station 503. According to this, thespreading code switching unit 624 sends a signal to a spreading codesequence generating unit 604 to change over the spreading code, and thefrequency switching unit 626 sends a signal to the frequency converter606 to change over the output frequency.

The transmission unit of the base station and the reception unit of themobile station are similar in construction to the transmission unit andthe reception unit as shown in FIGS. 1A and 1B. Further, the receptionunit of the base station is similar in construction to the receptionunit of the mobile station as shown in FIG. 1B, with the frequencyswitching unit 162 and the spreading code switching unit 164 removed.

FIG. 7 shows transmission signals of the base station 502 undercommunication and the new base station 503 during the hand-off period.During the hand-off period, under the control of the upper level station501, in the two base stations 502 and 503 making hand-off operation,error correction code is not produced, but only the user data sequenceportion is modulated to make communication from both the communicatingbase station 502 and the new base station 503. Specifically, the maincontroller 109 (FIG. 1A) of each base station enables or disables theerror correction encoder 102 according to the instruction from the upperlevel station 501 to transmit the user data in the timing as shown inFIG. 7.

On the other hand, in the mobile station 504, the reception frequency ischanged over in the course of the frame to receive and demodulate thetransmission signals from the communicating base station 502 and the newbase station 503. The two signals received during the period of thechange-over are stored in a memory (not shown) which are read andcombined to make decision. This reproduces reliable user data even atthe change-over.

Transmission timing from the mobile station when making hand-off isshown in FIG. 8. In the mobile station, the user data is primarymodulated and secondary modulated to be transmitted. When makinghand-off, during the hand-off period, error correction code is notproduced, but only the user data is frequency changed over in the courseof the frame and transmitted two times. The transmission signal is shownin FIG. 8(A). In this case, as shown in FIG. 8(B), the same user data ischanged over in transmission frequency and transmitted to thecommunicating base station 502 and the new base station 503. Thecommunicating base station 502 and the new base station 503 individuallyreceive, demodulate, and make decision on the transmission data andtransmit the detected data to the upper level station. A reproductionunit 510 of the upper level station 501 selects one of the two detectedsignals to reproduce a reliable user data. At this moment, one of higherreception level can be selected. Using this method, hand-off is possiblewithout degradation of reception quality and without instantaneousinterception. Alternatively, the demodulated signal from thecommunicating base station 502 and the demodulated signal from the newbase station may be combined to make decision on the user data from themobile station.

EMBODIMENT 4

Hand-off operation to change over the base station during communicationusing the same apparatus in making level monitoring described in FIGS.3A, 3B, 4A and 4B will be described with reference to FIGS. 5, 9, and10. FIG. 9 shows the construction of transmitter of a mobile station,and FIG. 10 shows hand-off timing.

FIG. 9 is a block diagram showing the construction of a transmissionunit of the mobile station 504. In the figure, symbol 90X (X=1-9)corresponds to symbol 30X in FIG. 3A.

A difference of this transmission unit from the transmission unit of thebase station as shown in FIG. 3A is that the former has a sub-controller920. The sub-controller 920 comprises a modulated level switcher 922, aspreading code switching unit 924, and a frequency switching unit 926,and these units are supplied with control information from a frameforming unit 901. The modulation level switcher 922 sends a signal to amodulation level controller 904 to control the modulation level. Thatis, in normal mode, the user data and the error correction check bitsare outputted in a period of 1 frame, whereas in hand-off mode, as shownin FIG. 10, the modulation level is controlled so that the user data andthe error correction check bits are outputted in a period of 1/2 frame.According to this, the spreading code switching unit 924 sends a signalto a spreading code sequence generating unit 906 to change over thespreading code, and the frequency switching unit 926 sends a signal to afrequency converter 907 to change over the output frequency.

The transmission unit of the base station and the reception unit of themobile station are similar in construction to the transmission unit andthe reception unit shown in FIGS. 3A and 3B. Further, the reception unitof the base station is similar in construction to the reception unit ofthe mobile station as shown in FIG. 3B, with the frequency switchingunit 362 and the spreading code switching unit 364 removed.

Hand-off in the present embodiment is carried out by changing themodulation of primary modulation. Both the communicating base station502 and the new base station 503, during the hand-off period, under thecontrol of the upper level station, as shown in FIG. 10, changes themodulation level of primary modulation to make a blank time. Here,unlike hand-off in the third embodiment, it is not necessary to removethe error correction code, and the user data and the error correctioncheck bits are transmitted.

The mobile station 504 changes over the station to be received in thehand-off period. At the receiving side, the reception frequency ischanged over in the course of 1 frame to achieve hand-off. Signals ofthe two changed-over base stations 502, 503 are received anddemodulated, the demodulated signals are stored in a memory (not shown),read and combined. The combined signal, as a signal of 1 frame, is errorcorrection decoded to reproduce the transmission data.

On the other hand, in hand-off of the transmitting side in the mobilestation 504, the mobile station 504 modulates the transmission datausing a changed modulation level, changes over the transmissionfrequency in the course of 1 frame, and transmits two times. The twobase stations 502, 503 related to hand-off error correction decode thereceived signal to reproduce the transmission data to be transferred tothe upper level station 501. The upper level station 501 selects eitherof the data received and reproduced by the communicating base station orthe new station. At this time, for example, one of higher receptionlevel may be selected.

EMBODIMENT 5

In the case of using measurement described in FIGS. 1A and 1B and FIG.2, another system of hand-off for changing over the base station duringcommunication using the same apparatus will be described with referenceto FIGS. 5 and 11. FIG. 11 shows transmission timing at the base stationwhen hand-off is carried out

In this case, the transmitter of the base station and the receiver ofthe mobile station are similar to those shown in FIGS. 1A and 1B.Further, the transmitter of the mobile station and the receiver of thebase station are also similar to those shown in FIGS. 1A and 1B.

The communicating base station 502, as shown in FIG. 11, does notproduce error correction code under control of the upper level station501 during the hand-off period, but modulates only the user datasequence portion for transmission. From the new base station 503, onlythe error correction check bits are modulated and transmitted.

The mobile station 504 changes over the reception frequency in thecourse of frame, and receives the user data from the communicating basestation 502 and the error correction encoded check bits from the newbase station 503. Then, the data from both base stations is stored in amemory, read to reproduce the original 1 frame data. Since the errorcorrection check bits are received from the new base station 503, thereproduced 1 frame data can be error correction decoded, therebyreproducing more reliable transmission data.

EMBODIMENT 6

In the case of using measurement described in FIGS. 1A and 1B and FIG.2, a further system of hand-off for changing over the base stationduring communication using the same apparatus will be described withreference to FIGS. 5 and 12. FIG. 12 shows transmission timing at themobile station when hand-off is carried out.

In this case, the transmitter of the base station and the receiver ofthe mobile station are similar to those shown in FIGS. 1A and 1B.Further, the transmitter of the mobile station and the receiver of thebase station are also similar to the transmitter shown in FIG. 6 and thereceiver shown in FIG. 1B.

The mobile station 504, as shown in FIG. 12, during the hand-off period,modulates the user data and the error correction check bits andtransmits them in alternation. That is, in the first half of frame, theuser data is transmitted at the reception frequency fo of thecommunicating base station 502, and in the second half of the frame, theerror correction check bits are transmitted at the reception frequencyfk of the new base station 503.

The communicating base station 502 receives the user data of the firsthalf of the frame, and the new base station 503 receives the errorcorrection coding check bits of the second half of the frame. Thereproduction unit 510 of the upper level station 501 stores the datafrom both base stations, which is read to reproduce the original 1 framedata. In this case, the upper level station 501 receives the errorcorrection coding check bits from the new base station 503, thereproduced 1 frame data can be error correction decoded, therebyreproducing more reliable transmission data.

What is claimed is:
 1. A CDMA mobile communications method for makingtransmission by providing a mobile station, a first base station to beconnected to said mobile station via the radio using a first frequency,a second base station to be connected to said the mobile station via theradio using a second frequency different from said first frequency, andan upper level station to be connected to said first and second basestations, said method comprising the steps of:at each of said first andsecond base stations,inputting user data to be transmitted to saidmobile station, and error correction encoding said user data, therebygenerating error correction check bits; generating a narrow-bandmodulated signal by primary modulation of said user data and said errorcorrection check bits; generating a wideband signal by secondarymodulation of said narrow band modulated signal using a spreading code;and transmitting said wideband signal; and at said mobile station,receiving said wideband signal; restoring a narrow-band modulated signalby primary demodulation of said wideband signal using a spreading codeidentical to said spreading code; restoring user data and errorcorrection check bits by secondary demodulation of said narrow-bandmodulated signal; reproducing user data by error correction decodingsaid user data and said error correction check bits, wherein at saidmobile station, said wideband signal transmitted from said first basestation is received at said first frequency in a first time in whichsaid user data is to be transmitted, and said user data is reproduced;and changing over a reception frequency to said second frequency tomeasure reception level of a signal transmitted from said second basestation in a second time in which said error correction check bits is tobe transmitted.
 2. The CDMA mobile communication method as claimed inclaim 1, wherein said mobile station has a function of making a hand-offoperation in which communication is changed over from said first saidbase station to said second base station, and during the hand-offoperation, said mobile station concurrently communicates with either ofsaid first base station and said second base station,said first basestation comprises a step for transmitting said user data in the firsttime and discontinuing the transmission in the second time, said secondbase station comprises a step for transmitting said user data in thesecond time and discontinuing the transmission in the first time, saidmobile station comprises a step for receiving and demodulating a signalfrom said first base station in the first time, changing over thefrequency of a receiver to the radio frequency of said second basestation in the second time to receive and demodulate the signal fromsaid second base station, and reproducing said user data by combiningthe signal from said first base station and the signal from said secondbase station.
 3. The CDMA mobile communication method as claimed inclaim 2, wherein, during the hand-off operation,said mobile stationcomprises a step for transmitting said user data to said first basestation at a first radio frequency in a third time in which said userdata is to be transmitted, and transmitting said user data to saidsecond base station at a second radio frequency in a fourth time inwhich said error correction check bits are to be transmitted, said firstbase station comprises a step for demodulating said user data receivedin the third time, said second base station comprises a step fordemodulating said user data received in the fourth time, and said upperlevel station of said first base station and said second base stationcomprises a step for reproducing said user data by combining thesedemodulated signals.
 4. The CDMA mobile communication method as claimedin claim 1, wherein said mobile station has a function of making ahand-off operation in which communication is changed over from saidfirst said base station to said second base station, and during thehand-off operation, said mobile station concurrently communicates witheither of said first base station and said second base station,saidfirst base station comprises a step for transmitting said user data inthe first time and discontinuing the transmission in the second time;said second base station comprises a step for transmitting said errorcorrection check bits in the second time and discontinuing thetransmission in the first time; said mobile station comprises a step forreceiving and demodulating a signal from said first base station in thefirst time, changing over the frequency of a receiver to the radiofrequency of said second base station in the second time to receive anddemodulate the signal from said second base station, and reproducingsaid user data by using said user data from said first base station anderror correction check bits from said second base station.
 5. The CDMAmobile communication method as claimed in claim 4, wherein, during thehand-off operation,said mobile station comprises a step for transmittingsaid user data at the first radio frequency to said first base stationin the third time in which said user data is to be transmitted, andtransmitting said error correction check bits at the second radiofrequency to said second base station in the fourth time in which saiderror correction check bits are to be transmitted; said first basestation comprises a step for demodulating said user data received in thethird time; said second base station comprises a step for demodulatingsaid error correction check bits received in the fourth time; and anupper level station of said first base station and said second basestation comprises a step for reproducing said user data by errorcorrection decoding using these demodulated signals.
 6. A CDMA mobilecommunication method for making transmission by providing a mobilestation, a first base station to be connected to said mobile station viathe radio using a first frequency, and a second base station to beconnected to said mobile station via the radio using a second frequencydifferent from said first frequency, said method comprising the stepsof:at each of said first and second base stations,inputting user data tobe transmitted to said mobile station, and error correction encoding thesaid user data, thereby generating error correction check bits;generating a narrow-band modulated signal by multilevel modulation ofsaid user data and said error correction check bits; generating awideband signal by spreading modulation of said narrow-band modulatedsignal using a spreading code; and transmitting said wideband signal;and at said mobile station,receiving said wideband signal; restoring anarrow-band modulated signal by spreading demodulation of said widebandsignal using a spreading code identical to said spreading code;restoring user data and error correction check bits by multileveldemodulation of said narrow-band modulated signal; and error correctiondecoding said user data and said error correction check bits, therebyreproducing user data: wherein at said mobile station, a multilevel ofsaid multilevel modulation is increased, and said user data and saiderror correction check bits are transmitted in a first time which isshorter than a total time length corresponding to a sum of the signaltime length of said user data and a signal time length of said errorcorrection check bits, while transmission is discontinued in a secondtime which is within a time length obtained by subtracting said firsttime from said total time length; and wherein at said mobile station,the multilevel of said multilevel modulation is increased in the firsttime, and thereafter, said user data and said error correction checkbits are received to reproduce said user data, while a receptionfrequency is changed over to said second frequency in the second time tomeasure reception level of a signal transmitted from said second basestation.
 7. The CDMA mobile communication method as claimed in claim 6,wherein said mobile station has a function of making a hand-offoperation in which communication is changed over from said first saidbase station to said second base station, and during the hand-offoperation period, said mobile station concurrently communicates witheither of said first base station and said second base station,saidfirst base station comprises a step for transmitting said user data anits error correction check bits in the first time; said second basestation comprises a step for transmitting said user data and its errorcorrection check bits in the second time; said mobile station comprisesa step for receiving said user data and said error correction check bitsfrom said first base station in the first time, changing over thereception frequency to the radio frequency of said second base stationto receive said user data and said error correction check bits in thesecond time, and reproducing said user data by combining the signal fromthe said first base station and the signal from said second base stationand by performing error correction.
 8. A CDMA mobile communicationsystem comprising a mobile station, a first base station to be connectedto said mobile station via the radio using a first frequency, and asecond base station, a first base station to be connected to said mobilestation via the radio using a second frequency different from said firstfrequency, wherein each of said first and second base stationscomprise:means for inputting user data to be transmitted to said mobilestation, and error correction encoding said user data, therebygenerating error correction check bits; means for generating narrow-bandmodulated signal by primary modulation of said user data and said errorcorrection check bits; and means for generating a wideband signal bysecondary modulation of said narrow-band modulated signal using aspreading code, and transmitting said wideband signal; and wherein saidmobile station comprises:first means for receiving said wideband signal,and restoring a narrow-band modulated signal by primary demodulation ofsaid wideband signal using a spreading code identical to said spreadingcode; second means for restoring user data and error correction checkbits by secondary demodulation of said narrow-band modulated signal; andthird means for error correction decoding said user data and said errorcorrection check bits to reproduce user data; wherein said first meansof said mobile station sets reception frequency at said first frequencyto receive said wideband signal transmitted from said first base stationin a first time in which said user data is to be transmitted, and saidfirst means changes over the reception frequency to said secondfrequency to receive a signal transmitted from said second base stationin a second time in which said error correction check bits are to betransmitted; and said mobile station comprises:means for measuringreception level of the signal transmitted from said second base stationin the second time.
 9. The CDMA mobile communication system as claimedin claim 8 further comprising an upper level station for instructinghand-off between said first base station and said second base station,whereinsaid first data station comprises first control means fortransmitting said user data in the first time and discontinuing thetransmission in the second time according to the hand-off instructionfrom said upper level station; said second data station comprises secondcontrol means for transmitting said user data in the second time anddiscontinuing the transmission in the first time according to thehand-off instruction; and said mobile station comprises receivingfrequency switching means for changing the frequency of a receiverbetween the transmission frequency of said first base station and thetransmission frequency of said second base station, and a demodulatorfor receiving and demodulating the signal from said first base stationin the first time and receiving and demodulating the signal from saidsecond base station in the second time for combining the signal fromsaid first base station and the signal from said second base station toreproduce said user data.
 10. The CDMA mobile communication system asclaimed in claim 9, wherein the mobile station comprises transmissionfrequency change-over means for changing over the transmission frequencyof a transmitter between the reception frequency of said first basestation and the reception frequency of said second base station, andtransmission means for, during the hand-off period, transmitting userdata at the reception frequency of said first base station in the thirdtime in which said user data is to be transmitted and transmitting saiduser data at the reception frequency of said second base station in thefourth time in which said error correction check bits are to betransmitted;said first base station comprises a first demodulator fordemodulating said user data received in the third time; said second basestation comprises a second demodulator for demodulating said user datareceived in the fourth time; and said upper level station comprisesmeans for reproducing said user data by combining the demodulatedsignals demodulated by said individual demodulator.
 11. The CDMA mobilecommunication system as claimed in claim 8 further comprising an upperlevel station for instructing hand-off between said first base stationand said second base station, whereinsaid first base station comprisesfirst control means for transmitting said user data in the first timeand discontinuing the transmission in the second time according to thehand-off instruction from said upper level station; said second basestation comprises second control means for transmitting said errorcorrection check bits in the second time and discontinuing thetransmission in the first time according to the hand-off instruction;said mobile station comprises receiving frequency switching means forchanging the frequency of the receiver between the transmissionfrequency of said first base station and the transmission frequency ofsaid second base station, and a demodulator for receiving anddemodulating the signal from said first base station in the first timeand receiving and demodulating the signal from said second base stationin the second time to reproduce said user data using said user data fromsaid first base station and said error correction check bits from saidsecond base station.
 12. The CDMA mobile communication system as claimedin claim 11, whereinthe mobile station comprises transmission frequencychange-over means for changing over the transmission frequency of atransmitter between the reception frequency of said first base stationand the reception frequency of said second base station, andtransmission means for, during the hand-off period, transmitting userdata at the reception frequency of said first base station in the thirdtime in which said user data is to be transmitted and transmitting saiderror correction check bits at the reception frequency of said secondbase station in the fourth time in which said error correction checkbits are to be transmitted; said first base station comprises ademodulator for demodulating said user data received in the third time;said second base station comprises a demodulator for demodulating saiderror correction check bits received in the fourth time; and said upperlevel station comprises means for making error correction decoding usingthe demodulated signals outputted from said individual demodulator toreproduce said user data.
 13. A CDMA mobile communication systemcomprising a mobile station, a first base station to be connected tosaid mobile station via the radio using a first frequency, and a secondbase station to be connected to said mobile station via the radio usinga second frequency different from said first frequency, wherein each ofsaid first and second base stations comprises:means for inputting userdata to be transmitted to said mobile station, and error correctionencoding said user data, thereby generating error correction check bits;means for generating a narrow-band modulated signal by multilevelmodulation of said user data and said error correction check bits; andmeans for generating a wideband signal by spreading modulation of saidnarrow-band modulated signal using a spreading code, and transmittingsaid wideband signal; and said mobile station comprises:first means forreceiving said wideband signal, and restoring a narrow-band modulatedsignal by spreading demodulation of said wideband signal using aspreading code identical to said spreading code; second means forrestoring user data and error correction check bits by multileveldemodulation of said narrow-band modulated signal; and third means forreproducing user data by error correction decoding said user data andsaid error correction check bits; and wherein said first base stationcomprises a first modulation level control unit for varying themodulation level of said multilevel modulation of said user data andsaid error correction check bits, and means for making a second time fordiscontinuing transmission by transmitting with an increased modulationlevel in the first time during a reception level monitoring period forsaid mobile station to monitor the reception level of the signal from asecond base station differing in frequency from said first base station;and said mobile station comprises receiving frequency switching meansfor tuning the frequency of a receiver between the transmissionfrequency of said first base station and the transmission frequency ofsaid second base station, a second modulating level control unit forchanging the modulation level of said multilevel modulation, and ademodulator for demodulating the reception signal with an increasedmodulation level and making error correction decoding to reproduce saiduser data, and a received level measuring unit for changing over thereception frequency to the transmission frequency of said second basestation to monitor the reception level of said second base station. 14.The CDMA mobile communication system as claimed in claim 13, furthercomprising an upper level station for instructing hand-off between saidfirst base station and said second base station, whereinsaid first basestation comprises first transmission means for transmitting said userdata and its error correction check bits in the first time according tothe hand-off instruction from said upper level station; said second basestation comprises second transmission means for transmitting said userdata and its error correction check bits in the second time according tothe hand-off instruction; and said mobile station comprises means forreceiving said user data and said error correction check bits from saidfirst base station in the first time, changing over the receptionfrequency to the transmission frequency of said second base station inthe second time to receive said user data and said error correctioncheck bits, and combining the signal from said first base station andthe signal from said second base station to reproduce said user data.15. A CDMA mobile communication system comprising a mobile station, afirst base station to be connected to said mobile station via the radiousing a first frequency, and a second base station to be connected tosaid mobile station via the radio using a second frequency differentfrom said first frequency wherein each of said first and second basestations comprises:means for inputting user data to be transmitted tosaid mobile station, and error correction encoding said user datathereby generating error correction check bits; means for generating anarrow-band modulated signal by primary modulation of said user data andsaid error correction check bits; and means for generating a widebandsignal by secondary modulation of said narrow-band modulated signalusing a spreading code, and transmitting said wideband signal; and saidmobile station comprising a first base station which, after a user datais error correction encoded, transmits a narrow-band modulated signalobtained by primary modulation of said user data and an error correctioncheck bits, followed by secondary modulation by a spreading code to awideband signal, and a mobile station for making conversion of saidwideband signal to said narrow-band modulated signal, followed bydemodulation and error correction decoding, to reproduce said user data;wherein a receiver of said mobile station comprises: a demodulator fordemodulating said user data in a first time of receiving said user datatransmitted from said first base station; an error correction decoderwhich discontinues said error correction decoding in the second time ofreceiving said error correction check bits and outputs said user datawhich is not error correction decoded; a frequency switching unit forchanging over the reception frequency to a radio frequency of a secondbase station differing in used frequency from said first base station inthe second time; and a received level measuring unit for monitoring thereception level of said second base station.
 16. The mobile station asclaimed in claim 15, wherein said receiver of said mobile stationcomprising:a reception frequency switching unit for changing thereception frequency of said receiver between the transmission frequencyof said first base station and the transmission frequency of said secondbase station, and a demodulator which, during the hand-off periodbetween said first base station and said second base station, receivesand demodulates the signal from said first base station in the firsttime, receives and demodulates the signal from said second base stationin the second time for combining the signal from said first base stationand the signal from said second base station to reproduce said userdata; and said transmitter of said mobile station comprises:transmission frequency change-over means for changing the transmissionfrequency between the reception frequency of said first base station andthe reception frequency of said second base station; and transmissionmeans for, during the hand-off period, transmitting user data at thereception frequency of said first base station in the third time inwhich said user data is to be transmitted and transmitting said userdata at the reception frequency of said second base station in thefourth time in which said error correction check bits are to betransmitted; and wherein said first base station comprises a step fordemodulating said user data received in the third time; the second basestation comprises a step for demodulating said user data received in thefourth time; and a upper level station of said first base station andsaid second base station comprises a step for combining thesedemodulated signals to reproduce said user data.
 17. A CDMA mobilecommunication system comprising a mobile station, a first base stationto be connected to said mobile station via the radio using a firstfrequency, and a second base station to be connected to said mobilestation via the radio using a second frequency different from said firstfrequency wherein each of said first and second base stationscomprises:means for inputting user data to be transmitted to said mobilestation, and error correction encoding said user data thereby generatingerror correction check bits; means for generating a narrow-bandmodulated signal by multilevel modulation of said user data and saiderror correction check bits; and means for generating a wideband signalby spreading modulation of said narrow-band modulated signal using aspreading code, and transmitting said wideband signal; and said mobilestation comprises a first base station which, after a user data is errorcorrection encoded, transmits a narrow-band modulated signal obtained byprimary modulation with multilevel modulation of said user data and anerror correction check bits, followed by secondary modulation by aspreading code to a wideband signal, and said mobile station makingconversion of said wideband signal to said narrow-band modulated signal,followed by demodulation and error correction decoding, the reproducesaid user data, said first base station transmitting said user data andsaid error correction check bits in a first time with an increasedmultiple value of said multilevel modulation to make a second time fordiscontinuing transmission, wherein a receiver of said mobile stationcomprises: a modulated level switcher for increasing the modulationlevel of said multilevel modulation in the first time during receptionlevel monitoring period for monitoring the reception level of the signalfrom a second base station differing from said first base station; areception frequency switching unit for changing the reception frequencyof said receiver between the transmission frequency of said first basestation and the transmission frequency of a second base station; and areceived level measuring unit for monitoring the reception level of saidsecond base station in the second time.
 18. The mobile station asclaimed in claim 17, wherein said reception frequency switching unit ofsaid receiver of said mobile station, during a hand-off period betweensaid first base station and said second base station, changes over thereception frequency to the transmission frequency of said first basestation in the first time, and to the transmission frequency of saidsecond base station in the second time, and said means for combining thesignal from said first base station and the signal from said second basestation to reproduce said user data.